<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div class="div"></div>
</body>

</html>

<style>
    .div {
        font-size: 10em;
        display: inline-block;
        width: 0.4em;
        box-sizing: content-box;
        height: 0.4em;
        border: 0.1em solid red;
        position: relative;
        border-radius: 0.35em;
    }

    .div:before {
        content: "";
        display: inline-block;
        position: absolute;
        right: -0.25em;
        bottom: -0.1em;
        border-width: 0;
        background: red;
        width: 0.35em;
        height: 0.08em;
        transform: rotate(45deg);
    }
</style>